It was the secret terror that gripped astronaut Michael Collins throughout the Apollo 11 project 40 years ago. As his spacecraft, Columbia, swept over the lunar surface, Collins - the mission's third and largely forgotten crewman - waited for a call from fellow astronauts Neil Armstrong and Buzz Aldrin to say their lander craft had successfully blasted off from the Moon.

The message would banish Collins's deepest fear: that he would be the only survivor of an Apollo 11 disaster and that he was destined to return on his own to the United States as "a marked man".

The realisation that the normally icy-cool astronaut was so obsessed by such an outcome puts a fresh perspective on the celebrations that will, this weekend, absorb the United States as it commemorates the moment, on 21 July 1969, that an American first walked on another world. Apollo 11 will be presented as a flawless technological triumph at jamborees across the nation, including a special reception at the National Air and Space Museum in Washington, which all three Apollo 11 astronauts are scheduled to attend.

Yet at the time, worries that the mission would end in disaster consumed nearly all of those involved in the programme - despite their apparent calm. And no one was more stressed than Collins, it appears.

In his case, the astronaut was obsessed with the reliability of the ascent engine of Armstrong and Aldrin's lander, Eagle. It had never been fired on the Moon's surface before and many astronauts had serious doubts about its reliability. Should the engine fail to ignite, Armstrong and Aldrin would be stranded on the Moon - where they would die when their oxygen ran out. Or if it failed to burn for at least seven minutes, then the two astronauts would either crash back on to the Moon or be stranded in low orbit around it, beyond the reach of Collins in his mothership, Columbia.

All three astronauts believed there was a real chance such a disaster would occur. Armstrong thought his prospects were only 50-50 of making it back to Earth. And so did Collins, the pilot of Columbia and one of the world's most experienced aviators.

Nor were the astronauts alone. Richard Nixon, then US president, had even prepared a speech that he would deliver in the event of the Eagle's engine failing. "Fate has ordained that the men who went to the Moon to explore in peace will stay on the Moon to rest in peace," it ran. "These brave men, Neil Armstrong and Edwin Aldrin, know that there is no hope for their recovery. But they also know that there is hope for mankind in their sacrifice."

Thus Collins - alone in Columbia as the world focused on Armstrong and Aldrin walking on the lunar surface - fretted about his two companions below him on the Moon and revealed, in a note written at the time, that he was now "sweating like a nervous bride" as he waited to hear from the Eagle.

"My secret terror for the last six months has been leaving them on the Moon and returning to Earth alone; now I am within minutes of finding out the truth of the matter," he wrote. "If they fail to rise from the surface, or crash back into it, I am not going to commit suicide; I am coming home, forthwith, but I will be a marked man for life and I know it."

Then Armstrong and Aldrin prepared their lander for its launch. Armstrong pressed the engine's firing button and Eagle soared perfectly above the lunar surface towards the waiting Collins. His worst fear had not materialised and he returned safely to Earth in the company of Armstrong and Aldrin, unmarked by the experience. He would not suffer a fate of global notoriety.

In fact, the opposite happened. Collins was forgotten. Today most people still know the names of the two first men on the Moon and recall the words, delivered by Armstrong, about taking a giant leap for mankind. But the name Michael Collins is rarely recalled, despite his critical role in the historic flight of Apollo 11. Not that he holds grudges. "It was an honour," he said last week.

In fact, he was - in many ways - the unsung hero of the Apollo 11 mission, a point that was underlined at the time by the great American aviator Charles Lindbergh. He wrote to Collins, not long after his safe return, to tell him that his part of the mission was one of "greater profundity ... you have experienced an aloneness unknown to man before".

It is an intriguing remark and an apposite one, it turns out - a point that can be appreciated by looking at the very set-up of the mission. Apollo 11 consisted of a spindly lunar lander, Eagle, and an orbiting mothership, Columbia, that were both blasted into space on a giant Saturn V rocket on 16 July 1969. For three days, Armstrong, Aldrin and Collins cruised towards the Moon inside Columbia and spent their time gazing "out the window at the Earth getting smaller and smaller and checking the spacecraft", according to Aldrin.

Then, on 20 July, Armstrong and Aldrin crawled into Eagle and flew it down to the Moon's surface. "Keep talking to me, guys," radioed an initially panicky Collins as the pair drifted away from his ship.

Minutes later, Columbia swept behind the Moon and Collins became Earth's most distant solo traveller, separated from the rest of humanity by 250,000 miles of space and by the bulk of the Moon, which blocked all radio transmissions to and from mission control. He was out of sight and out of contact with his home planet.

"I am now truly alone and absolutely alone from any known life. I am it," he wrote in his capsule. Lindbergh's remarks were certainly accurate.

Such solitude would have unnerved most people. But not Collins. He says the emotion that he experienced most during his day alone in lunar orbit was that of exultation. And certainly he appears to have relished his time as the loneliest member of his species. He also emerged from the post-Apollo years relatively unscathed. Aldrin lapsed into alcoholism and depression, while Armstrong became a virtual recluse. Both men subsequently divorced. By contrast, Collins - shaded from the glare of publicity - has avoided such personal traumas and is still with his wife, Patricia, whom he married in 1958. The couple have three grown-up children.

Collins was born in Rome on 31 October 1930. His father, Major-General James Lawton Collins, was then serving overseas with the US army. Collins later graduated from West Point and joined the US air force. An early assignment was to the 21st Fighter-Bomber Wing at George Air Force Base, where he learned how to drop nuclear weapons. He joined the astronaut corps in 1962 and flew on one of America's two-man Gemini capsules with veteran astronaut John Young, who flew on a later Apollo mission. Then came his selection for Apollo 11.

After his return to Earth, Collins gave up space travel and pursued a career in bureaucracy and business. He was director of the National Air and Space Museum until 1978, before being appointed vice-president of LTV Aerospace in Arlington, Virginia. He resigned in 1985 to start his own business.

Today he remains cheerful about his role on Apollo 11, although he describes himself as becoming increasingly grumpy. "At age 78, some things about current society irritate me, such as the adulation of celebrities and inflation of heroism," he said last week. Neither description fits him, he added. "Heroes abound, but don't count astronauts among them. We worked very hard, we did our jobs to near perfection, but that is what we had been hired to do."

He describes himself today as moderately busy, "running, biking, swimming, fishing, painting, cooking, reading, worrying about the stock market and searching for a really good bottle of cabernet for under $10".

As to his claim to fame, that was simple fate, he added. "Neil Armstrong was born in 1930. Buzz Aldrin was born in 1930, and Mike Collins, 1930. We came along at exactly the right time. We survived hazardous careers and were successful in them.

"But in my own case at least, it was 10% shrewd planning and 90% blind luck. Put Lucky on my tombstone."